Egg sacks or sponges?

egg Sacks Sponges

Hello All! First let me say thanks for the info. and the website forums. I’m new to the hobby, love it, but have questions on a regular basis and everyone is soooo helpful! I have a newer 75G tank with lots of live rock (mushrooms, sponges, polyps, etc. on it), no invertebrates and only a few aggressive fish. In the morning when I first turn on the light, I noticed that overnight there is what I can best describe as something that reminds me of a bug egg sack growing on my one piece of granite rock, it is ovalish in shape, a bubble, not clear, kind of hazy…if I pull off the “skin” of the sack, underneath on my rock is what looks like grains of salt except they are grayish in color….what in the world is this stuff? Yesterday I pulled a piece of the skin off and this morning it is right back the way it was? Some people thought maybe it is a sponge